The CCD solid-state imaging device, which transfers signal charges from the photoelectric converter elements by a CCD transfer section, has a plurality of vertical transfer sections that transfer vertically the signal charges read out from the photoelectric converter elements arranged two-dimensionally in a matrix form, a horizontal transfer section that transfers horizontally the signal charges in an amount of one line transferred from the vertical transfer sections, and an output amplifier that converts the signal charges transferred by the horizontal transfer section into electric signal and output it.
The output amplifier generally includes a floating diffusion (hereinafter referred to as an FD) storing the signal charges transferred from the horizontal transfer section, and two source follower circuits connected in multiple stages and outputting a signal in accordance with a change in the potential on the FD (see Japanese Patent No. 2,986,752, for example). Each of the source follower circuits includes an enhancement drive transistor and a depression load transistor. In an effort to reduce the consumption power on the output amplifier configured as above, it is a practice in the related art to provide the drive transistor in the last-stage source follower circuit with a drain voltage sufficiently lower than the drain voltage of the drive transistor of the first-stage source follower circuit. Such a configuration is important particularly for a solid-state imaging device to mount on a cellular phone or the like because of the requirement of reduced power consumption.
In the output amplifier configured to realize reduced power consumption as above, because the drive transistor of the last-stage source follower circuit has a drain voltage sufficiently lower than that of the drive transistor of the first-stage source follower circuit, the drive transistor of the last-stage source follower circuit must be made as a transistor of a high enhancement type. Such a high-enhancement transistor can be realized by increasing the thickness of a gate oxide film (silicon dioxide film) of the transistor or raising the concentration of a P-type impurity, such as boron, to implant to the gate channel. However, making a transistor high in enhancement results in an increase in the gate(channel)-to-drain voltage of the transistor. The gate(channel)-to-drain voltage, if increased, reduces the transistor's gate modulation factor due to the short-channel effect. This results in a great gain reduction in the output amplifier overall.